Field of the Invention and Description of the Related Art
The present invention relates to an imaging apparatus having an illumination means.
Recently, electronic-type endoscopes (called electronic endoscopes or video endoscopes) in which solid-state imaging devices such as charge-coupled devices (hereafter abbreviated as CCDs) are used as imaging means have been put to practical use.
Since an imaging apparatus used in an endoscope is provided with an illumination means so as to shoot an image of an object in a dark place such as a somatic cavity, and imaging is effected under the illumination by the illumination means.
For instance, in the above-described endoscope, when examining the interior of a somatic cavity, an endoscopic image is set in a still mode to make a detailed observation, or a still image is recorded by a monitor imaging apparatus, an optical disk, or the like.
At that time, if an effective imaging period is long (e.g., 1/60 second), blurring occurs when the image is viewed as a still image. The shorter the distance between the tip of the endoscope and the wall of a somatic cavity (the distance of observation), the more this blurring is likely to occur.
As for this problem, as disclosed in U.S. Pat. No. 4,571,629, proposals have been made to shorten the effective imaging period to, for instance, 1/400 second or less by using a solid-state imaging device (hereafter referred to as the SID), thereby making it possible to prevent blurring.
In the prior art of aforementioned U.S. Pat. No. 4,571,629, a period of illumination is shortened by reducing the width of an opening provided in a rotary shutter so as to shorten the effective imaging period.
Incidentally, an element shutter is provided with a function (a reset function) in which charges accummulated in each pixel are not output as video signals and are extracted and discarded, and an interval between the reset and the read-out is set as required, thereby controlling the effective imaging period. As an example which is similar to this method, it is conceivable to provide an arrangement in which the SID itself is not provided with an element shutter, and a liquid crystal shutter is provided on the front of the SID, thereby shortening an exposure period (which is the effective imaging period in this case) with respect to the SID.
In the above-described U.S. Patent, the variable width of the opening can be set to only one half of the imaging period (1/60 second in the case of the NTSC system) at maximum, so that one half of the radiation power of the lamp is wasted. For that reason, if the distance of observation is long, there occurs trouble in which it becomes impossible to obtain an image of adequate brightness, such as a shortage in the amount of exposure of the SID.
Meanwhile, in the case of an electronic endoscope using an element shutter or a liquid crystal shutter, if a diagnosis is continued under strong illumination when the distance of observation is short, there occurs trouble due to thermic rays contained in the illumination, resulting in, for instance, a radiation burn, or thermal deformation or, in some cases, burning of an object in the case of an industrial camera. In this respect, no measure has been taken in the prior art.
Incidentally, there is an electronic endoscope which employs a method in which a mechanical diaphragm is detachably inserted at the front of a lamp. However, a shutter which operates at a high speed of 1/100 second or above cannot be realized, and the sound becomes disadvantageously large.
In addition, in an electronic endoscope in which a liquid crystal shutter is provided in front of the lamp, the liquid crystal shutter is liable to become damaged by the illumination light.
Furthermore, one using a flashing unit instead of a lamp has the drawbacks that the sound is large and anxiety is imparted to the patient.
Meanwhile, even if an attempt is made to shorten the charge-accumulating time, the signal-to-noise (S/N) ratio declines since the amount of charged accumulated also declines.